There has been a rapid growth in the market for linear low density polyethylene (LLDPE), particularly resin made under mild operating conditions, typically at pressures of 100 to 300 psi and reaction temperatures of less than 100.degree. C. This low pressure process provides a broad range of LLDPE products for blown and cast film, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. LLDPE has essentially a linear backbone with only short chain branches, about 2 to 6 carbon atoms in length. In LLDPE, the length and frequency of branching, and, consequently, the density, is controlled by the type and amount of comonomer used in the polymerization. Although the majority of the LLDPE resins on the market today have a narrow molecular weight distribution, LLDPE resins with a broad molecular weight distribution are available for a number of non-film applications.
LLDPE resins designed for commodity type applications typically incorporate 1-butene as the comonomer. The use of a higher molecular weight alpha-olefin comonomer produces resins with significant strength advantages relative to those of ethylene/1-butene copolymers. The predominant higher alpha-olefin comonomers in commercial use are 1-hexene, 4-methyl-1-pentene, and 1-octene. The bulk of the LLDPE is used in film products where the excellent physical properties and drawdown characteristics of LLDPE film makes this film well suited for a broad spectrum of applications. Fabrication of LLDPE film is generally effected by the blown film and slot casting processes. The resulting film is characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance.
LLDPE, however, due to an inherent molecular structure/melt rheology characteristic, is highly susceptible to melt fracture, a phenomenon, whereby, on exiting an extruder die, the extrudate has a highly irregular surface. LLDPE is particularly vulnerable to melt fracture at die rates in excess of about 4 pounds per hour per inch of die circumference and at die gaps in the range of about 0.020 to about 0.075 inch.